The purpose of an automobile transmission is to provide a neutral, at least one reverse and one or more forward driving ranges that impart power from an engine, and/or other power sources, to the driving wheels, as required to provide for the necessary driving force and the desired performance.
A planetary gear set consists of an inner gear member, or sun gear, an outer gear member, or ring gear, and a planetary carrier assembly which includes, and supports, a plurality of planet gear members. When the sun gear is held stationary and power is applied to the ring gear, the planet gear members rotate in response to the power applied to the ring gear and thus "walk" circumferentially about the fixed sun gear to effect rotation of the carrier assembly in the same direction as the direction in which the ring gear is being rotated.
When any two members of a planetary gear set rotate in the same direction and at the same speed, the third member is forced to turn at the same speed, and in the same direction. For example, when the sun gear and the ring gear rotate in the same direction, and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive.
However, when the two gear members rotate in the same direction, but at different speeds, the direction in which the third gear member rotates may often be determined simply by visual analysis, but in many situations the direction will not be obvious and can only be determined by knowing the number of teeth present in the gear members of the planetary gear set.
Whenever the carrier assembly is restrained from spinning freely, and power is applied to either the sun gear or the ring gear, the planet gear members act as idlers. In that way, the driven member is rotated in the opposite direction as the drive member. Thus, in many transmission arrangements when the reverse drive range is selected, a torque transfer device serving as a brake is actuated frictionally to engage the carrier assembly and thereby restrain it against rotation so that power applied to the sun gear will turn the ring gear in the opposite direction. Thus, if the ring gear is operatively connected to the drive wheels of a vehicle, such an arrangement is capable of reversing the rotational direction of the drive wheels, and thereby reversing the direction of the vehicle itself.
As those skilled in the art will appreciate, a transmission system using a power split arrangement will receive power from two sources. Utilization of one or more planetary gear sets permits tow or more gear trains, or modes, by example, efficient hydrostatic units cannot accept another shaft operating concentrically through their pump and motor shafts. Therefore, a hydrostatic pump and motor will not be on the transmission centerline, but on a parallel main shaft with external gearing to transfer power from the hydrostatic unit to the centerline of the transmission. As a result, a hydrostatic pump and motor are not compatible with a compact round concentric transmission design. Furthermore, hydrostatic units that operate at greater than 5,000 psi are very noisy.
One possible solution to the aforementioned problems, is to replace the hydro-mechanical transmission with an all-electric drive transmission. However, all-electric transmission units have their own peculiar problems. In particular, the all-electric units are inefficient because engine power is transformed into electrical power to drive a motor, and that electrical power is then transformed back into mechanical, rotary power by the motor to drive the vehicle. Thus, each transformation of mechanical to electrical power, and vice versa between the engine and the drive wastes horsepower.
Accordingly, there is a need in the art for a large, horsepower transmission system with a power split that can efficiently use both electrical and mechanical input in a two-mode arrangement.